JP2004218025A - High strength steel sheet having excellent workability and shape fixability, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet whose workability (particularly, uniform elongation) and shape fixability (particularly, a low yield ratio) can be increased in a high strength region and an ultrahigh strength region on a 500 to 1,400 MPa class, and to provide a method of securely producing the steel sheet. <P>SOLUTION: The high strength steel sheet has a composition containing, by mass, 0.06 to 0.6% C, 0.5 to 3% Si+Al, 0.5 to 3% Mn, ≤0.15% (exclusive of 0%) P and ≤0.02% (exclusive of 0%) S, respectively. The steel sheet has a structure consisting of tempered martensite in ≥15% by area to the whole structure, ferrite in 5 to 60% by area to the whole structure, and retained austenite in ≥5% by volume ratio to the whole structure, and capable of comprising bainite and/or martensite as well. Also, 2% strain is added to the retained austenite. Thus, the ratio of the retained austenite transformed into martensite reaches 20 to 50%, so that its workability and shape fixability can be increased. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【００４４】
飽和磁化測定における印加磁化（Ｈ_ｍ）は、３９８０００〜７９６０００Ａ／ｍ［５０００〜１００００エルステッド（Ｏｅ）］であることが好ましい。また飽和磁化量は、測定温度の変化に影響を受けやすいことから、室温で測定する場合には、例えば２３℃±３℃の範囲内で行うことが好ましい。
【００４５】
２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトは、後述する様に、第一の連続焼鈍工程において、Ａ_１点以上Ａ_３点以下の温度で加熱保持した後、引き続いてＡ_３点超の温度で加熱保持することによって確実に生成させることができる。
【００４６】
次に、本発明鋼板を構成する基本成分について説明する。以下、化学成分の単位はすべて質量％である。
【００４７】
Ｃ： ０．０６ 〜 ０．６ ％
Ｃは、高強度を確保し、且つ、残留オーステナイト量を確保するために必須の元素である。詳細には、オーステナイト相中に充分なＣ量を含み、室温でも所望のオーステナイト相を残留させるために重要な元素であり、全伸びを高めるのに有用である。この様な効果を得るには、Ｃを０．０６％以上含有する必要がある。好ましくは０．１０％以上である。但し、０．６％を超えて添加すると、その効果が飽和するのみならず、鋳造中への中心偏析などによる欠陥などが見られる。好ましくは０．２５％以下にするのが良い。
【００４８】
Ｓｉ＋Ａｌ： ０．５ 〜 ３ ％
ＳｉやＡｌは、残留オーステナイトが分解して炭化物が生成するのを有効に抑える元素である。また特にＳｉは、固溶強化元素としても有用である。この様な作用を有効に発揮させるためには、ＳｉやＡｌを合計で０．５％以上添加することが必要である。好ましくは０．７％以上、より好ましくは１％以上である。但し、上記元素を合計で、３％を超えて添加しても上記効果は飽和してしまい、経済的に無駄である他、多量に添加すると、熱間脆性を起こす為、その上限を３％とする。好ましくは２．５％以下、より好ましくは２％以下である。なお、ＳｉとＡｌは併用して用いても良いし、Ｓｉ単独またはＡｌ単独で用いても良い。
【００４９】
Ｍｎ： ０．５ 〜 ３ ％
Ｍｎは、オーステナイト相を安定化し、所望の残留オーステナイトを得るために必要な元素である。この様な作用を有効に発揮させるためには、０．５％以上添加することが必要である。好ましくは０．７％以上、より好ましくは１％以上である。但し、３％を超えて添加すると、鋳片割れが生じる等の悪影響が見られる。好ましくは２．５％以下、より好ましくは２％以下である。
【００５０】
Ｐ： ０．１５ ％以下（ ０ ％を含まない）
Ｐは、所望の残留オーステナイトを確保するのに有効な元素である。この様な作用を有効に発揮させるためには、０．０３％以上（より好ましくは０．０５％以上）添加することが推奨される。但し、０．１５％を超えて添加すると二次加工性が劣化する。より好ましくは０．１％以下である。
【００５１】
Ｓ： ０．０２ ％以下（ ０ ％を含む）
Ｓは、ＭｎＳ等の硫化物系介在物を形成し、割れの起点となって加工性を劣化させる元素であり、０．０２％以下に抑制する必要がある。好ましくは０．０１５％以下である。
【００５２】
本発明の鋼は上記成分を基本的に含有し、残部は実質的に鉄及び不純物であるが、その他、本発明の作用を損なわない範囲で、以下の許容成分を添加することができる。
【００５３】
Ｍｏ： １ ％以下（ ０ ％を含まない），Ｎｉ： ０．５ ％以下（ ０ ％を含まない），Ｃｕ： ０．５ ％以下（ ０ ％を含まない），Ｃｒ： １ ％以下（ ０ ％を含まない）の少なくとも一種
これらの元素は、鋼の強化元素として有用であると共に、残留オーステナイトの安定化や所定量の確保に有効な元素である。この様な作用を有効に発揮させる為には、Ｍｏ：０．０５％以上（より好ましくは０．１％以上）、Ｎｉ：０．０５％以上（より好ましくは０．１％以上）、Ｃｕ：０．０５％以上（より好ましくは０．１％以上）、Ｃｒ：０．０５％以上（より好ましくは０．１％以上）を、夫々添加することが推奨される。但し、Ｍｏ及びＣｒは１％、Ｎｉ及びＣｕは０．５％を超えて添加しても上記効果が飽和してしまい、経済的に無駄である。より好ましくはＭｏ：０．８％以下、Ｎｉ：０．４％以下、Ｃｕ：０．４％以下、Ｃｒ：０．８％以下である。
【００５４】
Ｔｉ： ０．１ ％以下（ ０ ％を含まない），Ｎｂ： ０．１ ％以下（ ０ ％を含まない），Ｖ： ０．１ ％以下（ ０ ％を含まない）の少なくとも一種
これらの元素は、析出強化及び組織微細化効果があり、高強度化に有用な元素である。この様な作用を有効に発揮させるためには、Ｔｉ：０．０１％以上（より好ましくは０．０２％以上）、Ｎｂ：０．０１％以上（より好ましくは０．０２％以上）、Ｖ：０．０１％以上（より好ましくは０．０２％以上）を、夫々添加することが推奨される。但し、いずれの元素も０．１％を超えて添加すると上記効果が飽和してしまい、経済的に無駄である。より好ましくはＴｉ：０．０８％以下、Ｎｂ：０．０８％以下、Ｖ：０．０８％以下である。
【００５５】
Ｃａ： ０．００３ ％以下（ ０ ％を含まない）及び／又はＲＥＭ： ０．００３ ％以下（ ０ ％を含まない）
Ｃａ及びＲＥＭ（希土類元素）は、鋼中硫化物の形態を制御し、加工性向上に有効な元素である。ここで、本発明に用いられる希土類元素としては、Ｓｃ、Ｙ、ランタノイド等が挙げられる。上記作用を有効に発揮させる為には、夫々、０．０００３％以上（より好ましくは０．０００５％以上）添加することが推奨される。但し、０．００３％を超えて添加しても上記効果が飽和してしまい、経済的に無駄である。より好ましくは０．００２５％以下である。
【００５６】
次に、上記要件を満足する鋼板を確実に製造できる方法について説明する。
【００５７】
本発明の要件を満足する鋼板は、少なくとも熱延工程、第一の連続焼鈍工程および第二の連続焼鈍工程またはめっき工程を経て製造される。
【００５８】
但し、熱延工程の条件は特に限定されず、通常実施される条件を適宜選択して採用することができる。本発明の製法は、熱延工程により所望の組織を確保するものではなく、熱延後に実施する第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程を制御することにより所望の組織を得るところに特徴があるからである。
【００５９】
従って本発明の製法では、熱延工程の後、第一の連続焼鈍工程に先立って冷延工程を含むものであっても構わず、この冷延工程の条件も上記熱延工程と同じく特に限定されない。
【００６０】
具体的には、上記熱延工程としては、Ａ_ｒ３点以上で熱延終了後、平均冷却速度約３０℃／ｓｅｃで冷却し、約５００〜６００℃の温度で巻取る等の条件を採用することができる。また、冷延工程では、約３０〜７０％の冷延率の冷間圧延を施すことが推奨される。勿論、これに限定する趣旨では決してない。
【００６１】
次に、熱延工程（または熱延工程後の冷延工程）で得られた鋼板を、第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程で熱処理する。
【００６２】
このうち第一の連続焼鈍（先の連続焼鈍）は、Ａ_１点以上Ａ_３点以下の温度で１０〜３００秒加熱保持する工程；引き続きＡ_３点超の温度で１０〜３００秒加熱保持する工程；及び３０℃／ｓｅｃ以上の平均冷却速度で、Ｍｓ点以下の温度まで冷却する工程を包含する。これらの条件は、所望の母相組織（Ｃ含量のバラつきのある焼入マルテンサイト）を得るために設定されたものであり、この工程の説明図を図２に示す。
【００６３】
第二の連続焼鈍（後の連続焼鈍）またはめっきは、Ａ_１点以上Ａ_３点以下の温度で１０〜６００秒加熱保持する工程；３℃／ｓｅｃ以上の平均冷却速度で、Ｍｓ点以上Ｂｓ点以下の温度まで冷却する工程；及び該温度域で１秒以上保持する工程を包含する。これらの条件は、前記第一の連続焼鈍工程で生成した母相組織（焼入マルテンサイト）を焼戻して所望の焼戻マルテンサイトを得ると共に、Ｃγ_Ｒが低く不安定な残留オーステナイトとＣγ_Ｒが高く安定な残留オーステナイトの割合が適切に制御された第２相を得るために設定されたものである。この工程の説明図を図３に示す。
【００６４】
以下、前記図２および図３を参酌しつつ、第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程について説明する。
【００６５】
第一の連続焼鈍工程
上記熱延工程（または熱延工程後の冷延工程）で得られた鋼板を、Ａ_１点以上Ａ_３点以下の温度（図２中、Ｔ１）に加熱して均熱処理する（好ましくは１３００℃以下）。均熱処理することによってフェライトを一部生成させて［フェライト＋オーステナイト］の２相組織とする。
【００６６】
Ａ_１点以上Ａ_３点以下の温度で加熱保持する時間（図２中、ｔ１）は、Ｃ含量の異なるオーステナイトを生成させるために極めて重要であり、１０〜３００秒とする必要がある。保持時間が１０秒未満では、所望量のオーステナイトが生成しない。好ましくは３０秒以上保持することが望ましい。しかし加熱保持時間が３００秒を超えて長くなると、未溶解炭化物を核とするオーステナイト相の生成が進行してしまい、これをＡ_３点超の温度に加熱保持してもオーステナイト中のＣ含量のバラツキが小さくなると考えられる。この様な観点から保持時間は３００秒以下にする必要があり、好ましくは１２０秒以下にするのが望ましい。
【００６７】
次に、上記２相組織とした鋼板を、引き続きＡ_３点超の温度（図２中、Ｔ２）に加熱し、フェライトをオーステナイト化する。Ａ_１点以上Ａ_３点以下の温度で均熱処理した鋼板を、さらにＡ_３点超に加熱するのは、前記フェライトをオーステナイト化することによって、上記Ａ_１点以上Ａ_３点以下の温度域で生成したオーステナイトよりも、Ｃ含量の少ないオーステナイトを生成させるためである。即ちＡ_１点以上Ａ_３点以下の温度域では、フェライトとオーステナイトの２相が生成するが、フェライトに含まれるＣ量は鋼中のＣ量よりも少ないので、過剰なＣはオーステナイト中へ拡散する。その結果、当該温度域で生成するオーステナイト中のＣ含量は、鋼中に含まれるＣ量よりも多くなる。これに対し、Ａ_３点超の温度域ではオーステナイトのみが生成するので、該温度域で生成するオーステナイト中のＣ含量は、鋼中に含まれるＣ量とほぼ等しくなる。
【００６８】
従って、Ａ_１点以上Ａ_３点以下の温度域で生成するオーステナイトよりも、Ａ_１点以上Ａ_３点以下の温度域で生成したフェライトをオーステナイト化したものの方がＣ含量は少なくなる。しかも本発明では、その後のＡ_３点超の温度域における加熱保持時間が適切に制御されているので、Ａ_３点超の温度では、Ｃ含量の異なるオーステナイトが存在することとなり、このオーステナイトを冷却することによって、Ｃ含量の異なる焼入マルテンサイトを生成できる。そして、この焼入マルテンサイトを後述する第二の連続焼鈍工程またはめっき工程において熱処理することで、最終的に得られる残留オーステナイトのＣ含量にバラツキが生じ、低降伏比化を達成できるのである。
【００６９】
ここで、Ａ_１点以上Ａ_３点以下の温度域で加熱保持した鋼板を、Ａ_３点超の温度で所定時間再加熱保持することによって、オーステナイト中のＣ含量にバラつきが生じる理由は、フェライトとオーステナイトの２相域（Ａ_１点以上Ａ_３点以下）では、オーステナイトはラスフェライト間に生成するのに対し、オーステナイト単相域（Ａ_３点超）では、未溶解炭化物が核となり、この核を中心にオーステナイト相が生成する。このためフェライトとオーステナイトの２相域と、オーステナイト単相域では、オーステナイトが生成するまでの時間に差が生じることとなり、この時間差が生成するオーステナイト中のＣ含量にバラつきをもたらすと考えられる。
【００７０】
なお、従来の鋼板では、熱延した鋼板（または熱延後に冷延した鋼板）をＡ_１点以上Ａ_３点以下の温度またはＡ_３点以上の温度に加熱保持された後、冷却されるが、この様に、Ａ_１点以上Ａ_３点以下の温度またはＡ_３点以上の温度のみで加熱保持したとしても、オーステナイト中のＣ含量にバラつきは生じない。従って、この様な製造条件では、本発明で規定する要件を満足する組織は得られないと考えられる。
【００７１】
Ａ_３点超の温度域で加熱保持する時間は、Ｃ含量の異なるオーステナイトを生成させるために極めて重要であり、１０〜３００秒とする必要がある。保持時間が１０秒未満では、Ａ_１点以上Ａ_３点以下の温度域で生成したフェライトがオーステナイト化しない。好ましくは３０秒以上、より好ましくは６０秒以上保持することが望ましい。しかし加熱保持時間が３００秒を超えて長くなると、Ｃの拡散が生じ、オーステナイト間でＣの均一拡散が進んでＣ含量が平均化してしまうため所望の効果を得ることができない。この様な観点から保持時間は３００秒以下にする必要があり、好ましくは１８０秒以下、より好ましくは１２０秒以下にするのが望ましい。
【００７２】
次に、Ａ_３点以上の温度で所定時間加熱保持した鋼板は、３０℃／ｓｅｃ以上の平均冷却速度（図２中、ＣＲ１）で、Ｍｓ点以下の温度（図２中、Ｔ３）まで冷却する。この冷却によってパーライト変態を避けながら、焼入マルテンサイトを得ることができる。このとき得られる焼入マルテンサイトは、Ｃ含量が不均一になっていると考えられる。オーステナイトから焼入マルテンサイトへの変態は、Ｃの拡散を伴わないので、Ｃ含量の不均一なオーステナイトを冷却することによって得られる焼入マルテンサイト中のＣ含量は、オーステナイト中のＣ含量とほぼ等しくなるからである。
【００７３】
上記平均冷却速度は、フェライトの生成のみならず、最終的に得られる残留オーステナイトの形態にも影響を与え、平均冷却速度が大きければ（好ましくは５０℃／ｓｅｃ以上）、ラス状を呈することとなり一層好ましい。尚、平均冷却速度の上限は特に限定されず、大きければ大きい程良いが、実操業レベルとの関係で、適切に制御することが推奨される。
【００７４】
Ｍｓ点は、次に示す計算式から算出できる。
Ｍｓ＝５６１−４７４×［Ｃ］−３３×［Ｍｎ］−１７×［Ｎｉ］−１７×［Ｃｒ］−２１×［Ｍｏ］
式中、［ ］は各元素の質量％である。
【００７５】
第二の連続焼鈍工程またはめっき工程
上記第一の連続焼鈍工程で得られた鋼板は、Ａ_１点以上Ａ_３点以下の温度（図３中、Ｔ４）で１０〜６００秒（図３中、ｔ４）均熱することにより、所望の組織（焼戻マルテンサイト）を生成させる（２相域焼鈍）。この様な温度範囲で焼鈍する理由は、上記温度（Ａ_３点の温度）を超えると、第一の連続焼鈍工程で得られた焼入マルテンサイトがオーステナイト化してしまい、一方、上記温度（Ａ_１点の温度）を下回ると、所望の残留オーステナイトが得られないからである。更に、上記加熱保持時間（ｔ４）の制御は、所望の組織を得るために特に重要である。１０秒未満では焼戻が不足し、所望の母相組織（焼戻マルテンサイト）が得られないからである。好ましくは２０秒以上、より好ましくは３０秒以上である。しかし、加熱保持時間が６００秒を超えると、焼戻マルテンサイトの特徴であるラス状組織が維持できなくなり、機械的特性が劣化する。好ましくは５００秒以下、より好ましくは４００秒以下である。
【００７６】
次いで、平均冷却速度（図３中、ＣＲ２）を、３℃／ｓｅｃ以上（好ましくは５℃／ｓｅｃ以上）に制御し、パーライト変態を避けながら、Ｍｓ点以上Ｂｓ点以下の温度（ベイナイト変態：図３中、Ｔ５）まで冷却し、引き続きこの温度域で１秒以上（好ましくは５秒以上：図３中、ｔ５）保持する（オーステンパ処理）。これにより、残留オーステナイトへのＣ濃縮を、多量に且つ極めて短時間に得ることができる。
【００７７】
ここで、平均冷却速度が上記範囲を下回ると、所望の組織が得られず、パーライト等が生成する。尚、その上限は特に規定されず、大きければ大きい程良いが、実操業レベルとの関係で、適切に制御することが推奨される。
【００７８】
上記工程のうち、特にオーステンパ処理温度（Ｔ５）は、所望の組織を確保して本発明の作用を発揮させるのに重要である。上記温度範囲（Ｍｓ点以上Ｂｓ点以下）に制御すれば、多量の残留オーステナイトが得られ、これにより、残留オーステナイトによるＴＲＩＰ効果が発揮される。これに対し、Ｍｓ点未満ではマルテンサイト相が存在し、一方、Ｂｓ点を超えるとベイナイト相が多量に増加する。
【００７９】
上記保持時間（ｔ５）の上限は特に限定されないが、オーステナイトがベイナイトに変態する時間を考慮すると、好ましくは３０００秒以下、より好ましくは２０００秒以下に制御することが推奨される。
【００８０】
ここで、Ｂｓ点は次に示す計算式から算出できる。
Ｂｓ＝８３０−２７０×［Ｃ］−９０×［Ｍｎ］−３７×［Ｎｉ］−７０×［Ｃｒ］−８０×［Ｍｏ］
式中、［ ］は各元素の質量％である。
【００８１】
本発明におけるめっき条件は特に限定されず、公知のものを採用すれば良い。なお、合金化処理条件も公知のものを採用すれば良く、例えば５５０℃×６０ｓｅｃ程度で行なえば良い。この温度域では、γ相はセメンタイトへ分解するが、この程度の時間であれば本発明の効果を損なわないことを本発明者らは確認している。
【００８２】
以上の様に、熱延工程、（必要に応じて冷延工程）、第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程を経て製造される鋼板は、本発明の要件を満足する組織を有しており、特に残留オーステナイトのＣ含量にはバラつきが生じている。従って、相対的にＣ含量の低い不安定な残留オーステナイトは、歪を加えることによって容易にマルテンサイトへ変態し、フェライトとの間における可動転位密度を高めることができ、低降伏比化（具体的には５０％以下）を達成できる。また、相対的にＣ含量の高い安定な残留オーステナイトは、歪を加えても殆ど変態せず、さらに該残留オーステナイトはラス間に存在して微細のため、変形に対して安定であり、延性（特に全伸び）を高めることがでる。
【００８３】
よって、本発明の高強度鋼板は、加工性および形状凍結性に優れているので、例えば自動車や産業用機械等にプレス成形して使用される鋼板として好適に用いることができる。
【００８４】
【実施例】
以下、本発明を実施例によって更に詳細に説明するが、下記実施例は本発明を限定する性質のものではなく、前・後記の趣旨に適合し得る範囲で適当に変更して実施することも可能であり、それらはいずれも本発明の技術的範囲に含まれる。
【００８５】
実施例１：成分組成の検討
本実施例では、成分組成を変化させた場合における機械的特性の影響について調べた。具体的には、表１に記載の成分組成からなる供試鋼（表中の単位は質量％）を転炉溶製し、実験用スラブを得、各スラブを１２００℃で３０分間加熱した後、仕上温度（ＦＤＴ）を８５０℃として熱延し、次いで３０℃／ｓｅｃの平均冷却速度で巻取り温度（ＣＴ）６００℃まで冷却して、板厚２．５ｍｍの熱延鋼板を得た。
【００８６】
得られた熱延鋼板をそのまま又は酸洗後に常法に従って冷延し、板厚１．２ｍｍの冷延鋼板を得た。
【００８７】
得られた冷延鋼板を、前記図２および図３に示したパターンで処理した。具体的には、図２に示す第一の連続焼鈍工程については、Ｔ１：８５０℃、ｔ１：６０秒、Ｔ２：９３０℃、ｔ２：６０秒、ＣＲ１：５０℃／ｓｅｃ、Ｔ３：室温、であり、図３に示す第二の連続焼鈍工程については、Ｔ４：８５０℃、ｔ４：１２０秒、ＣＲ２：２５℃／ｓｅｃ、Ｔ５：４２０℃、ｔ５：１２０秒である。これらの製造条件は本発明の要件を満足している。
【００８８】
なお、本実施例では、第一の連続焼鈍の後、さらに第二の連続焼鈍を行なった場合について示したが、本発明はこれに限定されるものではなく、第二の連続焼鈍の代わりに上記パターンでめっきを行なっても構わない。
【００８９】
第一の連続焼鈍工程および第二の連続焼鈍工程を経て得られた鋼板について、各組織の存在率を次に示す様に測定した。まず、鋼板をレペラー腐食し、透過型電子顕微鏡観察（倍率：１５０００倍）により組織を同定した後、光学顕微鏡観察（倍率：１０００倍）により顕微鏡写真から組織の面積率を測定した。各組織の面積率を表２に示す。
【００９０】
また、上記得られた鋼板について、残留オーステナイト全体のうち、２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合を、前掲した飽和磁化測定法で測定した。測定時における印加磁化（Ｈｍ）は３９８０００Ａ／ｍ［５０００エルステッド（Ｏｅ）］、測定温度は２３℃である。表２に、残留オーステナイト量（全体γ_Ｒ）および２％歪を加えることによりマルテンサイトへ変態する残留オーステナイト（変態γ_Ｒ）量を示すと共に、残留オーステナイト全体のうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合を夫々示す。
【００９１】
次に、第一の連続焼鈍工程および第二の連続焼鈍工程を経て得られた鋼板からＪＩＳ ５号試験片を作製し、引張試験によって引張強度（ＴＳ）、伸び（全伸びのこと：Ｅｌ）、及び降伏強度（ＹＰ）を測定した。測定されたＹＰとＴＳから降伏比（ＹＲ）を算出した。算出結果を表２に示す。なお、引張試験の歪速度は１ｍｍ／秒とした。また、Ｅｌは３５％以上を合格、ＹＲは５０％以下を合格とする。
【００９２】
【表１】

【００９３】
【表２】

【００９４】
これらの結果より、以下の様に考察できる。なお、以下Ｎｏ．は、全て表２中の実験Ｎｏ．を意味する。
【００９５】
Ｎｏ．２〜４、Ｎｏ．７〜１５は、いずれも本発明で規定する成分を満足する例であり、良好な全伸び（Ｅｌ）と低降伏比（ＹＲ）を有する高強度鋼板が得られた。従って、この高強度鋼板は加工性および形状凍結性に優れている。
【００９６】
これに対し、本発明で規定する成分のいずれかを満足しない下記例は、夫々以下の不具合を有している。
【００９７】
Ｎｏ．１は、Ｃ含量が少ない例であり、所望の残留オーステナイトが得られないためＥｌ（全伸び）が低い。
【００９８】
Ｎｏ．５は、（Ｓｉ＋Ａｌ）の合計含量が少ない例であり、所望の残留オーステナイトが得られないためＥｌ（全伸び）が低い。
【００９９】
Ｎｏ．６は、Ｍｎ含量が少ない例であり、所望の残留オーステナイトが得られないためＥｌ（全伸び）が低い。
【０１００】
実施例２：製造条件の検討
本実施例では、前記表１の鋼Ｎｏ．４の実験用スラブを用い、１１５０℃で３０分間の均質化焼鈍を行なったうえで仕上げ温度８５０℃の熱延して板厚３．２ｍｍとした後、冷延して板厚１．２ｍｍの冷延鋼板を得た。
【０１０１】
得られた冷延鋼板を、前記図２および図３に示したパターンで処理した。具体的な条件を表３に示す。
【０１０２】
第一の連続焼鈍工程、および第二の連続焼鈍工程またはめっき工程を経て得られた鋼板について、上記実施例１と同様に、各組織の存在率および残留オーステナイト全体のうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合を測定し、結果を表４に示す。また、上記実施例１と同様に引張試験を行い、引張強度（ＴＳ）、伸び（全伸びのこと：Ｅｌ）、及び降伏強度（０．２％耐力のこと：ＹＰ）を測定すると共に、降伏比（ＹＲ）を算出し、これらの結果を表４に示す。
【０１０３】
【表３】

【０１０４】
【表４】

【０１０５】
これらの結果より、以下の様に考察できる。なお、以下Ｎｏ．は、全て表４中の実験Ｎｏ．を意味する。
【０１０６】
Ｎｏ．１６およびＮｏ．３１は、いずれも本発明で規定する製造条件を満足する例であり、鋼の組成も本発明の要件を満足しているので、良好な全伸び（Ｅｌ）と低降伏比（ＹＲ）を有する高強度鋼板が得られた。従って、この高強度鋼板は加工性および形状凍結性に優れている。
【０１０７】
これに対し、本発明で規定する製造条件のいずれかを満足しない下記例は、夫々以下の不具合を有している。
【０１０８】
Ｎｏ．１７は、第一の連続焼鈍工程におけるＡ_３点超の温度での加熱保持時間（ｔ２）が長過ぎるため、Ｃの均一拡散が進行してオーステナイト中のＣ含量が均一となる。従って、最終的に得られる残留オーステナイト中のＣ含量も均一となるので、安定な残留オーステナイトの割合が多くなり、残留オーステナイトのうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回り、降伏比が高くなる。
【０１０９】
Ｎｏ．１８は、第一の連続焼鈍工程におけるＡ_３点超の温度での加熱保持時間（ｔ２）が短過ぎるため、Ａ_１点以上Ａ_３点以下の温度で生成したフェライトがオーステナイト化しない。よって、焼入マルテンサイトの量が少なくなり、最終的に得られる焼戻マルテンサイト量が少なくなるため、全伸びが低くなると共に、降伏比が高くなる。
【０１１０】
Ｎｏ．１９は、第一の連続焼鈍工程におけるＡ_１点以上Ａ_３点以下の温度を下回る範囲で加熱保持しているため、焼入マルテンサイト中のＣ含量が均一となっていると考えられ、最終的に得られる残留オーステナイト中のＣ含量が均一となる。そのため、残留オーステナイトのうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回り、降伏比が高くなる。
【０１１１】
Ｎｏ．２０は、第一の連続焼鈍工程におけるＡ_１点以上Ａ_３点以下の温度で加熱保持せず、Ａ_３点超の温度でのみ加熱保持した例である。従って、Ｃの均一拡散が進行してオーステナイト中のＣ含量が均一となり、最終的に得られる残留オーステナイト中のＣ含量が均一となる。よって、残留オーステナイトのうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回り、降伏比が高くなる。
【０１１２】
Ｎｏ．２１は、先願発明の方法を模した例であり、第一の連続焼鈍工程におけるＡ_１点以上Ａ_３点以下の温度でのみ加熱保持しており、Ａ_３点超の温度で加熱保持していない。従って、焼入マルテンサイトが少なくなるので、最終的に得られる焼戻マルテンサイトも少なくなる。また、残留オーステナイト中のＣ含量が均一となるため、残留オーステナイトのうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回るため、降伏比が高くなる。
【０１１３】
Ｎｏ．２２は、第一の連続焼鈍工程におけるＡ_３点超の温度からの平均冷却速度（ＣＲ１）が小さいため、パーライトが生成し、最終的に得られる焼戻マルテンサイトの量が少なくなる。従って、Ｅｌ（全伸び）が低く、且つ、降伏比も高い。
【０１１４】
Ｎｏ．２３は、第一の連続焼鈍工程における冷却後の温度（Ｔ３）がＭｓ点を超えているので、最終的に焼戻ベイナイトが生成する。よって、焼戻マルテンサイトが生成せず、降伏比が高くなる。
【０１１５】
Ｎｏ．２４は、第二の連続焼鈍工程における加熱保持温度（Ｔ４）がＡ_３点よりも高いため、最終的にベイナイトが多く生成し、焼戻しマルテンサイトが少なくなる。従って、延びが低くなる。
【０１１６】
Ｎｏ．２５は、第二の連続焼鈍工程におけるＡ_１点以上Ａ_３点以下の温度（Ｔ４）での加熱保持時間（ｔ４）が長過ぎるため、フェライトが多く生成し、焼戻マルテンサイト量が少なくなる。そのためＥｌ（全伸び）が低くなる。また、残留オーステナイトのうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が、本発明の要件を下回るので、降伏比が高くなる。
【０１１７】
Ｎｏ．２６は、第二の連続焼鈍工程におけるＡ_１点以上Ａ_３点以下の温度（Ｔ４）での加熱保持時間（ｔ４）が短過ぎるため、オーステナイトの生成量が少なく、残留オーステナイトが少なくなる。従って、Ｅｌ（全伸び）が低くなる。また、加熱保持時間（ｔ４）が短いので、試料全体の温度が８５０℃に達せず、フェライトの生成量が多くなる。
【０１１８】
Ｎｏ．２７は、第二の連続焼鈍工程におけるＡ_１点以上Ａ_３点以下の温度からの平均冷却速度（ＣＲ２）が小さいため、パーライトが多く生成する。従って、最終的に得られる焼戻マルテンサイトの量が少なくなり、Ｅｌ（全伸び）が低く、且つ、降伏比が高くなる。
【０１１９】
Ｎｏ．２８は、第二の連続焼鈍工程における冷却後の温度（Ｔ５）が高過ぎるため、パーライトが多く生成する。従って、最終的に得られる焼戻マルテンサイトの量が少なくなり、Ｅｌ（全伸び）が低く、且つ、降伏比が高くなる。
【０１２０】
Ｎｏ．２９は、第二の連続焼鈍工程における冷却後の温度（Ｔ５）が低過ぎるため、最終的に得られる残留オーステナイト量が１％と非常に少なく、Ｅｌ（全伸び）が低くなる。また、残留オーステナイトのうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトは少ないので、降伏比が高くなる。
【０１２１】
Ｎｏ．３０は、第二の連続焼鈍工程におけるＡ_１点以上Ａ_３点以下の温度（Ｔ４）で加熱保持した後、この温度域からＭｓ点未満の温度まで急冷しているので、不安定な残留オーステナイトが多く、残留オーステナイトのうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が大きくなるので、破断し易くなり、Ｅｌ（全伸び）が低く、且つ、降伏比が高くなる。
【０１２２】
Ｎｏ．３２は、第一の連続焼鈍工程におけるＡ_１点以上Ａ_３点以下の温度（Ｔ１）での加熱保持時間（ｔ１）が長過ぎるため、未溶解炭化物を核とするオーステナイト相の生成が進行してしまい、これをＡ_３点超の温度に加熱してもオーステナイト中のＣ含量のバラツキが小さくなると考えられる。従って、最終的に得られる残留オーステナイト中のＣ含量も均一となるので、安定な残留オーステナイトの割合が多くなり、残留オーステナイトのうち２％歪を加えることによりマルテンサイトへ変態する残留オーステナイトの割合が本発明の範囲を下回り、降伏比が高くなる。
【０１２３】
【発明の効果】
本発明によれば、５００〜１４００ＭＰａ級の高強度及び超高強度鋼板であっても、全伸びが３５％以上で、且つ、降伏比が５０％以下の加工性および形状凍結性に優れた高強度鋼板およびその鋼板を確実に製造できる方法を提供できた。
【図面の簡単な説明】
【図１】飽和磁化測定に用いる装置の一部分を示す概略説明図である。
【図２】第一の連続焼鈍工程を説明した図である。
【図３】第二の連続焼鈍工程またはめっき工程を説明した図である。
【符号の説明】
１ 磁化検出用の４πＩコイル
２ 棒状試料
３ 磁界検出用のＨコイル
４ 導線[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength steel sheet, and more particularly, to a high-strength steel sheet having both workability and shape freezing property in a high-strength range of 500 to 1400 MPa and an ultra-high strength range, and a method for producing the same.
[0002]
[Prior art]
Steel sheets used for press forming in automobiles, industrial machines, and the like are required to have both excellent strength and ductility, and such required characteristics have been increasing more and more in recent years.
[0003]
However, as the strength of the steel sheet increases, there is a problem that springback (elastic recovery) after processing the steel sheet increases and shape freezing property deteriorates. An index of the shape freezing property includes a yield stress (0.2% proof stress) in a tensile test, and the lower the yield stress, the better the shape freezing property. Since high strength steel sheets are required to have high maximum strength, the ratio between the yield stress and the maximum strength (that is, the yield ratio) needs to be low (for example, see Non-Patent Document 1).
[0004]
On the other hand, in order to improve workability during press forming, it is necessary to increase the ductility of a steel sheet, and in particular, it is required to improve the total elongation.
[0005]
As a technique aiming to provide a steel sheet having such a low yield ratio and a large total elongation, for example, Patent Document 1 specifies the component composition of a cold-rolled steel sheet and changes the structure of the steel sheet to the residual austenite, martensite, and ferrite. There has been proposed a technique for improving the properties of a cold-rolled steel sheet such as strength, weldability, ductility, low yield ratio, and bake hardenability by forming a composite structure. However, although the cold-rolled steel sheet obtained by this technique achieves an elongation of about 35%, its yield ratio is about 55%, and further improvement in shape freezing properties is desired.
[0006]
[Non-patent document 1]
“Steel Sheet Forming Technology Research Group”, “Press Forming Difficult Handbook”, Nikkan Kogyo Shimbun, July 31, 1997 (second edition) p. See 178
[Patent Document 1]
JP-A-3-77743 (refer to [Claims], Table 2, page 8)
[0007]
[Problems to be solved by the invention]
The present invention has been made in view of such a situation, and its object is to provide workability (especially full elongation) and shape freezing (especially low elongation) in a high-strength range and an ultra-high-strength range of 500 to 1400 MPa. An object of the present invention is to provide a high-strength steel sheet excellent in yield ratio) and a method for reliably manufacturing the steel sheet.
[0008]
[Means for Solving the Problems]
The high-strength steel sheet excellent in workability and shape freezing property according to the present invention, which can solve the above problems, is, by mass%, C: 0.06 to 0.6%, Si + Al: 0.5 to 3%. , Mn: 0.5-3%, P: 0.15% or less (excluding 0%), S: 0.02% or less (including 0%), and tempered martensite: all 15% or more in area ratio to the structure, ferrite: 5 to 60% in area ratio to the whole structure, retained austenite: 5% or more in volume ratio to the whole structure, further containing bainite and / or martensite The gist is that the ratio of the retained austenite that transforms to martensite by applying a 2% strain to the retained austenite is 20 to 50%.
[0009]
In the present invention, further by mass%,
(1) Mo: 1% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), Cu: 0.5% or less (excluding 0%), Cr: 1% Containing at least one of the following (not including 0%):
(2) Ti: at least one of 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), V: 0.1% or less (not including 0%) Containing,
(3) Ca: 0.003% or less (excluding 0%) and / or REM: 0.003% or less (excluding 0%)
thing,
Are preferred embodiments.
[0010]
The method for reliably producing a high-strength steel sheet according to the present invention includes at least a hot rolling step, a first continuous annealing step, and a second continuous annealing step or a plating step, wherein the first continuous annealing step is A₁A above the point₃Step of heating and holding at a temperature below the point for 10 to 300 seconds;₃A step of heating and holding at a temperature above the point for 10 to 300 seconds; and a step of cooling to a temperature of not more than the Ms point at an average cooling rate of 30 ° C./sec or more, wherein the second continuous annealing step or the plating step is performed. , A₁A above the point₃A step of heating and holding at a temperature not higher than the point for 10 to 600 seconds; a step of cooling to a temperature of not lower than the Ms point and not higher than the Bs point at an average cooling rate of 3 ° C./sec or higher; The point is that it has the gist.
[0011]
In the present invention, it is preferable that a cold rolling step is included after the hot rolling step and before the first continuous annealing step.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
In order to provide a high-strength steel sheet having a large total elongation and a low yield ratio, the present inventors have proposed a TRIP steel sheet [retained austenite (γ)_R), And a steel plate which undergoes transformation (strain-induced transformation: TRIP) of retained austenite during work deformation to improve ductility], with particular attention paid to retained austenite. As a result, after the TRIP steel sheet is hot-rolled (cold-rolled if necessary) and then continuously annealed, the first continuous annealing (the previous continuous annealing) is referred to as “A”.₁A above the point₃After heating and holding at a temperature below the point (two-phase region) for a predetermined time, A₃Heating and holding at a temperature higher than the point (austenite single phase region) for a predetermined time ”, and the second continuous annealing (subsequent continuous annealing) or plating is performed using“ A₁A above the point₃A high-strength steel sheet having both of these characteristics by heat treatment as a configuration that “heats and holds at a temperature below the temperature (two-phase region) for a predetermined time” [specifically, the matrix structure is a mixture of tempered martensite and ferrite. In the TRIP steel sheet, the second phase is a retained austenite structure, and the retained austenite that transforms to martensite by applying a 2% strain to the retained austenite (hereinafter, such retained austenite (γ)_R), Γ_RC concentration (Cγ_R) May be referred to as “unstable retained austenite”, which is low), and the steel sheet is controlled strictly. Hereinafter, the basic concept (basic concept) of the steel sheet of the present invention will be described in detail while describing the history of reaching the present invention.
[0013]
First, regarding ductility (particularly total elongation), which is one of the required properties in the present invention, it is generally considered that retained austenite governs ductility in the structure of TRIP steel sheet, and further improves ductility. To achieve this, Cγ_RIt is said that it is useful to increase the stability of the retained austenite by increasing the austenite.
[0014]
On the other hand, with respect to the shape freezing property (especially low yield ratio) which is another required property in the present invention, martensite existing in the initial stage of deformation or unstable retained austenite such as martensite transformation near the yield point is dominant. It is said to be.
[0015]
However, the conventional TRIP steel sheet has Cγ from the viewpoint of emphasizing the improvement of ductility._RThe fact is that martensite and unstable retained austenite, which are useful for lowering the yield ratio, are extremely reduced because the concentration is increased to increase the stability of retained austenite. As a result, although the local ductility is very excellent, the yield ratio is high, and the stretch formability is poor.
[0016]
In order to provide a TRIP steel sheet having a low yield ratio while securing excellent ductility, the inventors of the present invention have proposed a new TRIP steel sheet previously applied [using a mixed structure of tempered martensite and ferrite as a parent phase, Studies have been made on the basis of the structure of a high-strength steel sheet having fine retained austenite as a phase (Japanese Patent Application No. 2002-54606, hereinafter referred to as the prior invention). By using a steel sheet having a unique structure as in the above-mentioned prior invention, a high-strength steel sheet having an extremely excellent strength-ductility balance could be provided. (Approximately 60%) because there is still a problem of poor shape freezing property.
[0017]
As a result, if the above-described heat treatment unique to the present invention is adopted, Cγ_RThe present inventors have found that a predetermined amount of retained austenite having a different degree of stability (that is, retained austenite that transforms into martensite by applying 2% strain) can be obtained due to variations in concentration, and the present invention has been completed.
[0018]
The reason for obtaining a high-strength steel sheet having a low yield ratio of 50% or less while maintaining excellent elongation by the present invention is that, in order to generate a desired retained austenite, particularly in the prior invention, This is because the annealing process has been improved.
[0019]
That is, both of the present invention and the prior invention are common in that the TRIP steel sheet is manufactured through hot rolling (cold rolling as necessary) → first continuous annealing → second continuous annealing. Since the first continuous annealing step is immediately cooled after heating and holding at the two-phase zone temperature, the temperature is raised to the austenite single-phase zone and cooled after heating and holding at the two-phase zone temperature as in the present invention. It is considered that the amount of C in martensite (quenched martensite) generated in this step is larger than that in the case where the heat treatment is performed. As a result, according to the prior invention, an austenite phase having a high C concentration and a high stability is obtained, and the C concentration (Cγ) in the residual austenite generated by the second continuous annealing is obtained._R) Is high, and since the retained austenite is present in a fine morphology between the laths, it is stable against deformation. As a result, unstable retained austenite which transforms from retained austenite to martensite cannot be obtained. It is considered that a low yield ratio could not be realized.
[0020]
On the other hand, according to the present invention, in performing the first continuous annealing, first, after heating to the two-phase region temperature, the temperature is raised to the austenite single-phase region, and then cooled. Austenite having a lower C content than in the case is generated between lath ferrites. Moreover, according to the present invention, since the heating time in the austenite single phase region is appropriately controlled (as short as possible), there is a time difference until undissolved carbides become nuclei to form austenite. The C content in the austenitic material varies. Therefore, an austenitic phase having a different C concentration is generated in the steel, and as a result, quenched martensite generated by the first continuous annealing also has a variable C content. Become.
[0021]
Furthermore, according to the present invention, since the heat treatment is appropriately performed in the two-phase region also in the subsequent second continuous annealing step, the C content (Cγ) of the finally generated residual austenite is obtained._R) Is also likely to vary. As a result, Cγ_RA predetermined amount of unstable retained austenite is obtained, and such retained austenite undergoes martensitic transformation after heat treatment or at an early stage of strain, and the mobile dislocation density increases around ferrite. It is considered that the ratio (YR ≦ 50%) can be secured.
[0022]
The basic idea for obtaining the steel sheet of the present invention has been described above. Hereinafter, requirements that characterize the present invention will be described specifically.
[0023]
First, the structure that characterizes the steel sheet of the present invention will be described.
[0024]
In the present invention, a mixed structure of tempered martensite and ferrite is used as a matrix, and the tempered martensite is 15% or more in area ratio with respect to the whole structure, and the ferrite is 5 to 60% in area ratio with respect to the whole structure. Must be included. The reason for defining such a range is as follows.
[0025]
Tempered martensite: Area ratio to total structure Fifteen %that's all
The term “tempered martensite” in the present invention means a soft structure having a low dislocation density and a lath shape. On the other hand, "martensite" differs from the above-mentioned tempered martensite in that it is a hard structure having a large dislocation density, and both can be distinguished by, for example, observation with a transmission electron microscope (TEM). The conventional retained austenite steel sheet also differs from the steel sheet of the present invention using tempered martensite as a parent phase in that it has a soft block-like ferrite structure having a low dislocation density.
[0026]
As described later, tempered martensite having such characteristics is obtained by converting a hot-rolled steel sheet (or a cold-rolled steel sheet after hot rolling) in the first continuous annealing step.₁A above the point₃After heating and holding at a temperature below the point,₃Heating and holding at a temperature above the point, and quenching from the temperature to produce quenched martensite, and in the second continuous annealing step or plating step, A₁Above the point (above about 700 ℃)₃It is obtained by annealing at a temperature below the point.
[0027]
The tempered martensite is necessary to increase the total elongation of the steel sheet and lower the yield ratio, and its content is 15% or more (preferably 20% or more) in terms of area ratio with respect to the entire structure. It is important that The content of tempered martensite is determined by the balance with retained austenite, which will be described later, and may be appropriately controlled within a range in which desired characteristics can be exhibited, but is preferably about 65% or less, more preferably. Is 35% or less.
[0028]
Ferrite: Area ratio for all structures 5 ~ 60 %
In the present invention, "ferrite" means polygonal ferrite, that is, ferrite having a low dislocation density, and elongation characteristics (particularly, total elongation) can be enhanced by containing this ferrite. In order to exert such an effect, it is necessary to contain ferrite in an area ratio of 5% or more (preferably 10% or more) with respect to the whole structure. However, if it exceeds 60%, it becomes difficult to secure the required strength, so the upper limit is set to 60%. More preferably, it is desirably 50% or less.
[0029]
The calculation procedure of the area ratio of each structure (excluding retained austenite) is as follows. For ferrite and martensite, optical microscope observation is performed by repeller corrosion, and for bainite, optical microscopic observation is performed by picral corrosion, and the area ratio of each structure is calculated from the photograph. However, in the optical microscope observation by Repeller corrosion, martensite and retained austenite cannot be distinguished. Therefore, in the present invention, from the total area ratio (area%) of martensite and retained austenite calculated by optical microscope observation by Repeller corrosion, The value obtained by subtracting the amount of retained austenite (% by volume) measured by the saturation magnetization measurement method described below is defined as the amount of martensite (area%).
[0030]
In the present invention, it is necessary to use retained austenite as the second phase and to contain the retained austenite in a volume ratio of 5% or more with respect to the entire structure. The reason for limiting to such a range is as follows.
[0031]
Retained austenite (γ _R ): By volume ratio for all tissues 5 %that's all
Retained austenite is useful for improving the total elongation and further the fatigue properties. In order to effectively exert such an effect, the retained austenite must be present in a volume ratio of 5% (preferably 7% or more) with respect to the entire structure. is necessary. The content of retained austenite is determined by the balance with the above-mentioned tempered martensite and ferrite, and may be appropriately controlled within a range in which desired characteristics can be exhibited, but is preferably about 30% or less. Preferably it is 25% or less. The method for calculating the volume fraction of retained austenite will be described later.
[0032]
The steel sheet of the present invention may have bainite and / or martensite as the second phase structure in addition to the retained austenite as long as the action of the present invention is not impaired. This is because this structure can inevitably remain in the manufacturing process of the present invention. The content is not particularly limited, and the smaller the better, the better. However, it is recommended that the area ratio be controlled to preferably 5% or less, more preferably 3% or less with respect to the whole tissue.
[0033]
In the present invention, the retained austenite which transforms to martensite by applying a 2% strain to the retained austenite (ie, Cγ_RIt is important that the ratio of low and unstable residual austenite) is 20 to 50%.
[0034]
When the ratio of retained austenite that transforms to martensite by applying 2% strain is less than 20% with respect to the entire retained austenite, the ratio of stable retained austenite increases to 80% or more. Even when% strain is applied, retained austenite hardly transforms into martensite. As a result, an increase in the mobile dislocation density due to the transformation cannot be expected, and the yield ratio remains high. However, when the ratio is 20% or more and the amount of unstable retained austenite in the entire retained austenite is large, the unstable retained austenite is easily transformed into martensite by applying a 2% strain, so , The movable dislocation density at the interface with ferrite is increased. As a result, a low yield ratio of the steel sheet can be realized. However, if the ratio exceeds 50%, the amount of unstable retained austenite becomes too large and the amount of stable retained austenite contributing to the improvement of ductility becomes insufficient, and high ductility of the steel sheet cannot be realized. Therefore, the proportion needs to be 50% or less (preferably 35% or less).
[0035]
As described above, in the present invention, the ratio of retained austenite that transforms to martensite by applying 2% strain to the entire retained austenite is controlled within the above range, so that stable retained austenite and unstable retained austenite And the balance of the amounts is good. As a result, the effect of improving ductility (particularly total elongation) by stable retained austenite and the effect of improving low yield ratio by unstable retained austenite are obtained in a well-balanced manner, and a low yield ratio and a large total elongation can be achieved.
[0036]
Here, the ratio of retained austenite that transforms to martensite by applying 2% strain to the entire retained austenite can be measured, for example, by the saturation magnetization measurement method previously proposed by the present inventors (Japanese Patent Application No. 2001-285750). reference). Hereinafter, the measurement principle and the measurement procedure of the saturation magnetization measurement method will be described.
[0037]
The saturation magnetization measurement method is based on the following measurement principle. That is, structures such as a ferrite phase and a martensite phase in a metal structure show ferromagnetism at room temperature, whereas an austenite phase is paramagnetic. Therefore, the saturation magnetization (Is) per unit volume of a metal structure composed of only a ferromagnetic structure such as a ferrite phase or a martensite phase is determined in advance, and the saturation magnetization (I) of a sample containing an austenite phase is measured. By doing so, the ratio of the austenite (γ) phase can be determined from the following equation (1). FIG. 1 schematically shows a part of an apparatus used for saturation magnetization measurement. The saturation magnetization is determined by generating a magnetic field using an electrode stone, mounting a 4πI coil 1 for magnetization detection and an H coil 3 for magnetic field detection between electromagnets, inserting a rod-shaped sample 2 into the 4πI coil 1, and closing the magnet. A magnetization curve is measured and determined in a state where a path is formed and the influence of the demagnetizing field is eliminated. In addition, 4 in FIG. 1 is a conducting wire.
γ (volume%) = (1−I / Is) × 100 (1)
[0038]
Next, the measurement procedure of the saturation magnetization measurement method will be described.
[0039]
A plurality of test pieces of 4 mm x 30 mm are cut out from the steel type to be measured, and these are superimposed to produce a columnar test piece having a thickness of about 4 mm. The saturation magnetization (I) of the columnar test piece and the saturation magnetization (Is) when the amount of retained austenite is substantially the same as that of the columnar test piece and the amount of retained austenite is 0% are measured or calculated. The residual austenite in the columnar test piece (hereinafter referred to as “whole γ”_R) May be calculated).
Overall γ_R(Volume%) = (1−I / Is) × 100 (2)
[0040]
Next, a tensile test piece is prepared from the steel type to be measured, strained by 2% by a tensile tester, and then a columnar test piece (4 mm × 3 mm × about 4 mm thick) is cut out from a parallel portion of the tensile test piece. From the saturation magnetization (It) and the saturation magnetization (Is) of the columnar test piece, residual austenite (hereinafter, referred to as “2% strain”) in the columnar test piece after applying 2% strain according to the following equation (3). After γ_R) May be calculated).
Γ after 2% strain_R(Volume%) = (1−It / Is) × 100 (3)
[0041]
The total γ calculated in this way_RAnd γ after 2% strain_RFrom residual austenite that transforms to martensite by applying 2% strain (hereinafter referred to as “transformation γ_R) May be calculated by the following equation (4).
Transformation γ_R(Volume%) = whole γ_RΓ after -2% strain_R    ... (4)
[0042]
Therefore, the entire retained austenite (overall γ_R), Retained austenite (transformation γ) transformed to martensite by applying 2% strain_R) Can be calculated by the following equation (5).
[0043]
(Equation 1)

[0044]
Applied magnetization (H_m) Is preferably 398000 to 796000 A / m [5000 to 10000 Oersted (Oe)]. In addition, since the saturation magnetization is easily affected by a change in the measurement temperature, when the measurement is performed at room temperature, it is preferable to perform the measurement, for example, within a range of 23 ° C. ± 3 ° C.
[0045]
Retained austenite, which is transformed into martensite by applying a 2% strain, causes A in the first continuous annealing step as described later.₁A above the point₃After heating and holding at a temperature below the point,₃By heating and holding at a temperature higher than the point, it can be surely generated.
[0046]
Next, basic components constituting the steel sheet of the present invention will be described. Hereinafter, all the units of the chemical components are% by mass.
[0047]
C: 0.06 ~ 0.6 %
C is an essential element for ensuring high strength and for securing the amount of retained austenite. Specifically, the element contains a sufficient amount of C in the austenite phase, is an important element for allowing the desired austenite phase to remain even at room temperature, and is useful for increasing the total elongation. In order to obtain such an effect, C must be contained at 0.06% or more. Preferably it is 0.10% or more. However, if the addition exceeds 0.6%, not only the effect is saturated, but also defects such as center segregation during casting are observed. Preferably, it is set to 0.25% or less.
[0048]
Si + Al: 0.5 ~ 3 %
Si and Al are elements that effectively suppress decomposition of residual austenite to form carbides. In particular, Si is also useful as a solid solution strengthening element. In order to effectively exert such an effect, it is necessary to add Si and Al in a total amount of 0.5% or more. It is preferably at least 0.7%, more preferably at least 1%. However, even if the total amount of the above elements exceeds 3%, the above effect is saturated and is economically useless. If a large amount is added, hot embrittlement is caused. And Preferably it is 2.5% or less, more preferably 2% or less. Note that Si and Al may be used in combination, or Si alone or Al alone may be used.
[0049]
Mn: 0.5 ~ 3 %
Mn is an element necessary for stabilizing the austenite phase and obtaining desired retained austenite. In order to effectively exert such an effect, it is necessary to add 0.5% or more. It is preferably at least 0.7%, more preferably at least 1%. However, if added in excess of 3%, adverse effects such as slab cracking are observed. Preferably it is 2.5% or less, more preferably 2% or less.
[0050]
P: 0.15 %Less than( 0 % Not included)
P is an element effective for securing desired retained austenite. In order to effectively exert such an effect, it is recommended to add 0.03% or more (more preferably, 0.05% or more). However, when added in excess of 0.15%, the secondary workability is deteriorated. It is more preferably at most 0.1%.
[0051]
S: 0.02 %Less than( 0 %including)
S is an element that forms sulfide-based inclusions such as MnS and serves as a starting point of cracks to deteriorate the workability, and needs to be suppressed to 0.02% or less. Preferably it is 0.015% or less.
[0052]
The steel of the present invention basically contains the above components, and the balance is substantially iron and impurities. However, other allowable components described below can be added within a range that does not impair the function of the present invention.
[0053]
Mo: 1 %Less than( 0 %), Ni: 0.5 %Less than( 0 %), Cu: 0.5 %Less than( 0 %), Cr: 1 %Less than( 0 % Not included)
These elements are useful elements for strengthening steel, and are effective elements for stabilizing retained austenite and securing a predetermined amount. In order to effectively exert such an effect, Mo: 0.05% or more (more preferably 0.1% or more), Ni: 0.05% or more (more preferably 0.1% or more), Cu : 0.05% or more (more preferably 0.1% or more), and Cr: 0.05% or more (more preferably 0.1% or more) are recommended to be added. However, even if Mo and Cr are added in amounts exceeding 1% and Ni and Cu exceed 0.5%, the above effects are saturated, which is economically useless. More preferably, Mo: 0.8% or less, Ni: 0.4% or less, Cu: 0.4% or less, Cr: 0.8% or less.
[0054]
Ti: 0.1 %Less than( 0 %), Nb: 0.1 %Less than( 0 %), V: 0.1 %Less than( 0 % Not included)
These elements have the effect of strengthening precipitation and making the structure finer, and are useful elements for increasing the strength. In order to effectively exhibit such an effect, Ti: 0.01% or more (more preferably 0.02% or more), Nb: 0.01% or more (more preferably 0.02% or more), V: : It is recommended to add 0.01% or more (more preferably 0.02% or more), respectively. However, if any of the elements is added in excess of 0.1%, the above-mentioned effects are saturated, which is economically useless. More preferably, Ti: 0.08% or less, Nb: 0.08% or less, and V: 0.08% or less.
[0055]
Ca: 0.003 %Less than( 0 %) And / or REM: 0.003 %Less than( 0 % Not included)
Ca and REM (rare earth elements) are elements that control the form of sulfide in steel and are effective for improving workability. Here, examples of the rare earth element used in the present invention include Sc, Y, and lanthanoid. In order to effectively exert the above effects, it is recommended to add 0.0003% or more (more preferably 0.0005% or more), respectively. However, even if it is added in excess of 0.003%, the above effect is saturated, which is economically useless. More preferably, it is 0.0025% or less.
[0056]
Next, a method for reliably producing a steel sheet satisfying the above requirements will be described.
[0057]
A steel sheet satisfying the requirements of the present invention is manufactured through at least a hot rolling step, a first continuous annealing step, and a second continuous annealing step or a plating step.
[0058]
However, the conditions of the hot rolling step are not particularly limited, and conditions usually performed can be appropriately selected and employed. The manufacturing method of the present invention does not secure a desired structure by a hot rolling step, but controls a first continuous annealing step to be performed after hot rolling, and a second continuous annealing step or a plating step to control a desired structure. This is because there is a characteristic in obtaining.
[0059]
Therefore, in the production method of the present invention, after the hot rolling step, the cold rolling step may be included prior to the first continuous annealing step, and the conditions of the cold rolling step are also particularly limited as in the above hot rolling step. Not done.
[0060]
Specifically, as the hot rolling step, A_r3After the completion of the hot rolling at the temperature above the temperature, cooling at an average cooling rate of about 30 ° C./sec and winding at a temperature of about 500 to 600 ° C. can be adopted. In the cold rolling step, it is recommended to perform cold rolling at a cold rolling rate of about 30 to 70%. Of course, this is by no means limiting.
[0061]
Next, the steel sheet obtained in the hot rolling step (or the cold rolling step after the hot rolling step) is subjected to heat treatment in a first continuous annealing step and a second continuous annealing step or a plating step.
[0062]
Of these, the first continuous annealing (the previous continuous annealing)₁A above the point₃Step of heating and holding at a temperature below the point for 10 to 300 seconds;₃A step of heating and holding at a temperature above the point for 10 to 300 seconds; and a step of cooling to a temperature below the Ms point at an average cooling rate of 30 ° C./sec or more. These conditions are set in order to obtain a desired matrix structure (quenched martensite having a variation in C content), and FIG. 2 is an explanatory diagram of this step.
[0063]
The second continuous annealing (subsequent continuous annealing) or plating is performed by A₁A above the point₃A step of heating and holding at a temperature not higher than the point for 10 to 600 seconds; a step of cooling to a temperature of not lower than the Ms point and not higher than the Bs point at an average cooling rate of 3 ° C./sec or higher; Include. Under these conditions, the matrix structure (quenched martensite) generated in the first continuous annealing step is tempered to obtain a desired tempered martensite, and Cγ_RLow and unstable retained austenite and Cγ_RThe ratio of the retained austenite having high and stable is set in order to obtain a properly controlled second phase. An explanatory diagram of this step is shown in FIG.
[0064]
Hereinafter, the first continuous annealing step and the second continuous annealing step or the plating step will be described with reference to FIGS.
[0065]
First continuous annealing process
The steel sheet obtained in the hot rolling step (or the cold rolling step after the hot rolling step) is₁A above the point₃Heating is performed to a temperature below the point (T1 in FIG. 2) (preferably 1300 ° C. or lower). The ferrite is partially generated by the soaking process to form a two-phase structure of [ferrite + austenite].
[0066]
A₁A above the point₃The time (t1 in FIG. 2) for heating and holding at a temperature below the point is extremely important for generating austenite having a different C content, and needs to be 10 to 300 seconds. If the holding time is less than 10 seconds, a desired amount of austenite will not be generated. Preferably, it is desirable to hold for 30 seconds or more. However, when the heating holding time is longer than 300 seconds, the formation of an austenite phase with undissolved carbide as a nucleus proceeds, which is caused by A₃It is considered that the variation in the C content in austenite is reduced even when the heating and holding are performed at a temperature higher than the point. From such a viewpoint, the holding time needs to be 300 seconds or less, preferably 120 seconds or less.
[0067]
Next, the steel sheet having the two-phase structure was continuously subjected to A₃The ferrite is heated to a temperature above the point (T2 in FIG. 2) to austenite the ferrite. A₁A above the point₃The steel sheet that has been soaked at a temperature below the₃Heating beyond the point is achieved by turning the ferrite into austenite, and₁A above the point₃This is because austenite having a lower C content is generated than austenite generated in a temperature range below the temperature. That is, A₁A above the point₃In the temperature range below the point, two phases of ferrite and austenite are formed, but since the amount of C contained in ferrite is smaller than the amount of C in steel, excess C diffuses into austenite. As a result, the C content in austenite generated in the temperature range becomes larger than the C content contained in the steel. In contrast, A₃Since only austenite is generated in a temperature range above the point, the C content in austenite generated in the temperature range is substantially equal to the C content contained in steel.
[0068]
Therefore, A₁A above the point₃Austenite formed in the temperature range below the point₁A above the point₃The austenite ferrite produced in the temperature range below the point has a lower C content. Moreover, in the present invention, the following A₃Since the heating holding time in the temperature range above the point is appropriately controlled, A₃At temperatures above the point, austenites having different C contents are present, and by cooling this austenite, quenched martensite having different C contents can be produced. By subjecting the quenched martensite to a heat treatment in the second continuous annealing step or plating step described later, the C content of the finally obtained retained austenite varies, and a low yield ratio can be achieved.
[0069]
Where A₁A above the point₃The steel sheet heated and held in the temperature range below the₃The reason why the C content in the austenite varies due to the reheating and holding for a predetermined time at a temperature exceeding the point is that the two-phase region of ferrite and austenite (A₁A above the point₃Austenite is formed between lath ferrites, whereas austenite single phase region (A₃In the case where the temperature exceeds the point, undissolved carbides become nuclei, and an austenite phase is formed around the nuclei. Therefore, there is a difference in the time until austenite is formed between the two-phase region of ferrite and austenite and the single-phase region of austenite, and it is considered that this time difference causes variation in the C content in the generated austenite.
[0070]
In the conventional steel sheet, a hot-rolled steel sheet (or a cold-rolled steel sheet after hot rolling) is referred to as A.₁A above the point₃Temperature below point or A₃After being heated and maintained at a temperature equal to or higher than the point, it is cooled.₁A above the point₃Temperature below point or A₃Even if the heating and holding are performed only at a temperature equal to or higher than the point, the C content in the austenite does not vary. Therefore, it is considered that a structure satisfying the requirements specified in the present invention cannot be obtained under such manufacturing conditions.
[0071]
A₃The time of heating and holding in a temperature range above the point is extremely important for generating austenite having a different C content, and needs to be 10 to 300 seconds. If the holding time is less than 10 seconds, A₁A above the point₃Ferrite produced in the temperature range below the point does not turn into austenite. It is desirable to hold for 30 seconds or more, more preferably 60 seconds or more. However, if the heating holding time is longer than 300 seconds, C diffusion occurs, and uniform diffusion of C proceeds between austenites, and the C content is averaged, so that a desired effect cannot be obtained. From such a viewpoint, the holding time needs to be 300 seconds or less, preferably 180 seconds or less, and more preferably 120 seconds or less.
[0072]
Next, A₃The steel sheet heated and maintained at a temperature equal to or higher than the temperature for a predetermined time is cooled to a temperature equal to or lower than the Ms point (T3 in FIG. 2) at an average cooling rate of 30 ° C./sec or higher (CR1 in FIG. 2). By this cooling, quenched martensite can be obtained while avoiding pearlite transformation. It is considered that the quenched martensite obtained at this time has a non-uniform C content. Since the transformation from austenite to quenched martensite does not involve the diffusion of C, the C content in quenched martensite obtained by cooling austenite having a non-uniform C content is approximately equal to the C content in austenite. This is because they are equal.
[0073]
The average cooling rate affects not only the formation of ferrite but also the form of the finally obtained retained austenite. If the average cooling rate is high (preferably 50 ° C./sec or more), the lath will be exhibited. More preferred. Note that the upper limit of the average cooling rate is not particularly limited, and the larger the better, the better. However, it is recommended to appropriately control the average cooling rate in relation to the actual operation level.
[0074]
The Ms point can be calculated from the following calculation formula.
Ms = 561-474 * [C] -33 * [Mn] -17 * [Ni] -17 * [Cr] -21 * [Mo]
In the formula, [] is mass% of each element.
[0075]
Second continuous annealing or plating process
The steel sheet obtained in the first continuous annealing step has A₁A above the point₃A desired structure (tempered martensite) is generated by soaking at a temperature equal to or lower than the point (T4 in FIG. 3) for 10 to 600 seconds (t4 in FIG. 3) (two-phase annealing). The reason for annealing in such a temperature range is as follows.₃When the temperature exceeds (point temperature), the quenched martensite obtained in the first continuous annealing step becomes austenite, while the temperature (A₁Below the temperature of the point), the desired retained austenite cannot be obtained. Further, the control of the heating holding time (t4) is particularly important for obtaining a desired tissue. If the time is less than 10 seconds, tempering is insufficient, and a desired matrix structure (tempered martensite) cannot be obtained. It is preferably at least 20 seconds, more preferably at least 30 seconds. However, when the heating holding time exceeds 600 seconds, the lath-like structure characteristic of tempered martensite cannot be maintained, and the mechanical properties deteriorate. Preferably it is 500 seconds or less, more preferably 400 seconds or less.
[0076]
Next, the average cooling rate (CR2 in FIG. 3) is controlled to 3 ° C./sec or more (preferably 5 ° C./sec or more), and the temperature between the Ms point and the Bs point (bainite transformation: The temperature is cooled to T5 in FIG. 3 and then maintained in this temperature range for 1 second or more (preferably 5 seconds or more: t5 in FIG. 3) (austempering). Thereby, a large amount of C can be obtained in the retained austenite in a very short time.
[0077]
Here, if the average cooling rate is lower than the above range, a desired structure cannot be obtained, and pearlite or the like is generated. The upper limit is not particularly defined, and the larger the better, the better. However, it is recommended to appropriately control the relationship with the actual operation level.
[0078]
Among the above steps, the austempering temperature (T5) is particularly important for securing a desired structure and exerting the effect of the present invention. When the temperature is controlled within the above-mentioned temperature range (not less than the Ms point and not more than the Bs point), a large amount of retained austenite is obtained, whereby the TRIP effect by the retained austenite is exhibited. In contrast, a martensite phase exists below the Ms point, while a bainite phase increases in a large amount above the Bs point.
[0079]
Although the upper limit of the holding time (t5) is not particularly limited, it is recommended to control it to preferably 3000 seconds or less, more preferably 2000 seconds or less in consideration of the time required for austenite to transform to bainite.
[0080]
Here, the Bs point can be calculated from the following calculation formula.
Bs = 830-270 * [C] -90 * [Mn] -37 * [Ni] -70 * [Cr] -80 * [Mo]
In the formula, [] is mass% of each element.
[0081]
The plating conditions in the present invention are not particularly limited, and known conditions may be adopted. It should be noted that known alloying conditions may be employed, for example, at about 550 ° C. × 60 sec. In this temperature range, the γ phase is decomposed into cementite, but the present inventors have confirmed that the effect of the present invention is not impaired for such a time.
[0082]
As described above, the steel sheet manufactured through the hot rolling step, (if necessary, the cold rolling step), the first continuous annealing step, and the second continuous annealing step or the plating step satisfies the requirements of the present invention. In particular, the C content of the retained austenite varies. Therefore, unstable retained austenite having a relatively low C content is easily transformed into martensite by applying strain, the mobile dislocation density between ferrite and ferrite can be increased, and a lower yield ratio (specifically, 50% or less). In addition, stable retained austenite having a relatively high C content hardly transforms even when strain is applied. Further, since the retained austenite is present between the laths and is fine, it is stable against deformation and has ductility ( In particular, the total elongation) can be increased.
[0083]
Therefore, the high-strength steel sheet of the present invention is excellent in workability and shape freezing property, and thus can be suitably used as a steel sheet to be press-formed and used in, for example, automobiles and industrial machines.
[0084]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are not intended to limit the present invention, and may be appropriately modified and implemented within a range that can conform to the purpose of the preceding and the following. It is possible, and they are all included in the technical scope of the present invention.
[0085]
Example 1: Examination of component composition
In this example, the influence of the mechanical properties when the component composition was changed was examined. Specifically, a test steel (unit: mass% in the table) having the component composition shown in Table 1 was melted in a converter to obtain experimental slabs, and each slab was heated at 1200 ° C. for 30 minutes. Then, hot rolling was performed at a finishing temperature (FDT) of 850 ° C., and then cooled to a winding temperature (CT) of 600 ° C. at an average cooling rate of 30 ° C./sec to obtain a hot-rolled steel sheet having a thickness of 2.5 mm.
[0086]
The obtained hot-rolled steel sheet was cold-rolled as it is or after pickling according to a conventional method, to obtain a cold-rolled steel sheet having a thickness of 1.2 mm.
[0087]
The obtained cold rolled steel sheet was treated in the pattern shown in FIGS. Specifically, in the first continuous annealing step shown in FIG. 2, T1: 850 ° C., t1: 60 seconds, T2: 930 ° C., t2: 60 seconds, CR1: 50 ° C./sec, T3: room temperature Yes, in the second continuous annealing step shown in FIG. 3, T4: 850 ° C., t4: 120 seconds, CR2: 25 ° C./sec, T5: 420 ° C., t5: 120 seconds. These manufacturing conditions satisfy the requirements of the present invention.
[0088]
Note that, in the present embodiment, after the first continuous annealing, the case where the second continuous annealing was further performed, but the present invention is not limited to this, instead of the second continuous annealing Plating may be performed with the above pattern.
[0089]
With respect to the steel sheet obtained through the first continuous annealing step and the second continuous annealing step, the existence ratio of each structure was measured as shown below. First, the steel plate was subjected to repeller corrosion, and the structure was identified by transmission electron microscope observation (magnification: 15000 times). Then, the area ratio of the structure was measured from a micrograph by optical microscope observation (magnification: 1000 times). Table 2 shows the area ratio of each tissue.
[0090]
Further, the ratio of the retained austenite that transforms to martensite by applying a 2% strain to the entire retained austenite of the obtained steel sheet was measured by the saturation magnetization measurement method described above. The applied magnetization (Hm) at the time of measurement is 398000 A / m [5000 Oersted (Oe)], and the measurement temperature is 23 ° C. Table 2 shows the amount of retained austenite (total γ)_R) And retained austenite transformed to martensite by applying 2% strain (transformation γ_R) And the ratio of retained austenite that transforms to martensite by applying a 2% strain to the total retained austenite.
[0091]
Next, a JIS No. 5 test piece was prepared from the steel sheet obtained through the first continuous annealing step and the second continuous annealing step, and the tensile strength (TS) and elongation (total elongation: El) were determined by a tensile test. , And yield strength (YP) were measured. The yield ratio (YR) was calculated from the measured YP and TS. Table 2 shows the calculation results. The strain rate in the tensile test was 1 mm / sec. El passes 35% or more, and YR passes 50% or less.
[0092]
[Table 1]

[0093]
[Table 2]

[0094]
From these results, the following can be considered. Note that the following No. Are all experimental Nos. In Table 2. Means
[0095]
No. Nos. 2 to 4; Nos. 7 to 15 are examples satisfying the components specified in the present invention, and high strength steel sheets having good total elongation (El) and low yield ratio (YR) were obtained. Therefore, this high-strength steel sheet is excellent in workability and shape freezing property.
[0096]
On the other hand, the following examples that do not satisfy any of the components specified in the present invention have the following disadvantages, respectively.
[0097]
No. No. 1 is an example in which the C content is small, and El (total elongation) is low because a desired retained austenite cannot be obtained.
[0098]
No. No. 5 is an example in which the total content of (Si + Al) is small, and El (total elongation) is low because a desired retained austenite cannot be obtained.
[0099]
No. No. 6 is an example in which the Mn content is small, and El (total elongation) is low because a desired retained austenite cannot be obtained.
[0100]
Example 2: Study of manufacturing conditions
In the present embodiment, the steel Nos. Using the experimental slab of No. 4 and performing homogenizing annealing at 1150 ° C. for 30 minutes, hot rolling at a finishing temperature of 850 ° C. to a sheet thickness of 3.2 mm, and then cold rolling to a sheet thickness of 1.2 mm. A cold rolled steel sheet was obtained.
[0101]
The obtained cold rolled steel sheet was treated in the pattern shown in FIGS. Table 3 shows specific conditions.
[0102]
For the steel sheet obtained through the first continuous annealing step and the second continuous annealing step or the plating step, a 2% strain is applied to the abundance ratio of each structure and the entire retained austenite in the same manner as in Example 1 above. The ratio of retained austenite that transforms to martensite was measured by the method described above, and the results are shown in Table 4. Further, a tensile test was performed in the same manner as in Example 1 to measure the tensile strength (TS), elongation (total elongation: El), and yield strength (0.2% proof stress: YP), and The ratio (YR) was calculated and the results are shown in Table 4.
[0103]
[Table 3]

[0104]
[Table 4]

[0105]
From these results, the following can be considered. Note that the following No. Are all experimental Nos. In Table 4. Means
[0106]
No. 16 and No. 31 is an example that satisfies the manufacturing conditions defined in the present invention, and the steel composition also satisfies the requirements of the present invention, so that it has a good total elongation (El) and a low yield ratio (YR). A high strength steel plate was obtained. Therefore, this high-strength steel sheet is excellent in workability and shape freezing property.
[0107]
On the other hand, the following examples that do not satisfy any of the manufacturing conditions specified in the present invention have the following disadvantages.
[0108]
No. 17 is A in the first continuous annealing step.₃Since the heating holding time (t2) at a temperature higher than the point is too long, the uniform diffusion of C proceeds and the C content in the austenite becomes uniform. Therefore, the C content in the finally obtained retained austenite is also uniform, so that the ratio of stable retained austenite increases, and the ratio of retained austenite that transforms to martensite by applying a 2% strain to the retained austenite is increased. Below the range of the present invention, the yield ratio increases.
[0109]
No. 18 is A in the first continuous annealing step.₃Since the heating holding time (t2) at a temperature above the point is too short, A₁A above the point₃Ferrite formed at temperatures below the point does not austenite. Therefore, the amount of quenched martensite decreases, and the amount of tempered martensite finally obtained decreases, so that the total elongation decreases and the yield ratio increases.
[0110]
No. 19 is A in the first continuous annealing step.₁A above the point₃Since the heating and holding are performed at a temperature lower than the temperature below the point, the C content in the quenched martensite is considered to be uniform, and the C content in the finally obtained residual austenite becomes uniform. Therefore, the ratio of retained austenite that transforms to martensite by applying 2% strain to the retained austenite falls below the range of the present invention, and the yield ratio increases.
[0111]
No. 20 is A in the first continuous annealing step.₁A above the point₃Without heating and holding at a temperature below the point₃This is an example of heating and holding only at a temperature higher than the point. Accordingly, the uniform diffusion of C proceeds to make the C content in the austenite uniform, and the C content in the finally obtained residual austenite becomes uniform. Therefore, the ratio of retained austenite that transforms to martensite by applying a 2% strain to the retained austenite falls below the range of the present invention, and the yield ratio increases.
[0112]
No. Reference numeral 21 denotes an example simulating the method of the invention of the prior application, in which A is used in the first continuous annealing step.₁A above the point₃Temperature only below the temperature₃Not heated and maintained at temperatures above the point. Therefore, since the amount of quenched martensite is reduced, the amount of tempered martensite finally obtained is also reduced. Further, since the C content in the retained austenite becomes uniform, the ratio of the retained austenite that transforms to martensite by applying a 2% strain to the retained austenite falls below the range of the present invention, so that the yield ratio increases.
[0113]
No. 22 is A in the first continuous annealing step.₃Since the average cooling rate (CR1) from a temperature above the point is small, pearlite is generated, and the amount of tempered martensite finally obtained is reduced. Therefore, El (total elongation) is low and the yield ratio is high.
[0114]
No. In No. 23, since the temperature (T3) after cooling in the first continuous annealing step exceeds the Ms point, tempered bainite is finally generated. Therefore, tempered martensite is not generated, and the yield ratio is increased.
[0115]
No. No. 24 indicates that the heating holding temperature (T4) in the second continuous annealing step is A₃Therefore, more bainite is finally formed and tempered martensite is reduced. Therefore, the elongation is low.
[0116]
No. 25 is A in the second continuous annealing step.₁A above the point₃Since the heating holding time (t4) at the temperature (T4) below the point is too long, a large amount of ferrite is generated and the amount of tempered martensite is reduced. Therefore, El (total elongation) decreases. Further, since the ratio of retained austenite that transforms to martensite by applying a 2% strain to the retained austenite falls below the requirement of the present invention, the yield ratio increases.
[0117]
No. 26 is A in the second continuous annealing step.₁A above the point₃Since the heating holding time (t4) at the temperature (T4) below the point is too short, the amount of generated austenite is small and the amount of retained austenite is small. Therefore, El (total elongation) decreases. Further, since the heating holding time (t4) is short, the temperature of the entire sample does not reach 850 ° C., and the amount of ferrite generated increases.
[0118]
No. 27 is A in the second continuous annealing step.₁A above the point₃Since the average cooling rate (CR2) from the temperature below the point is small, a large amount of pearlite is generated. Therefore, the amount of tempered martensite finally obtained is reduced, El (total elongation) is low, and the yield ratio is high.
[0119]
No. In No. 28, a large amount of pearlite is generated because the temperature (T5) after cooling in the second continuous annealing step is too high. Therefore, the amount of tempered martensite finally obtained is reduced, El (total elongation) is low, and the yield ratio is high.
[0120]
No. In No. 29, since the temperature (T5) after cooling in the second continuous annealing step is too low, the finally obtained residual austenite amount is as very small as 1%, and El (total elongation) becomes low. Further, since a small amount of the retained austenite is transformed into martensite by applying a 2% strain to the retained austenite, the yield ratio is increased.
[0121]
No. 30 is A in the second continuous annealing step.₁A above the point₃After heating and holding at a temperature below the temperature (T4), the temperature is rapidly cooled from this temperature range to a temperature below the Ms point, so that a large amount of unstable retained austenite is present. Since the ratio of retained austenite that transforms to 大 き く becomes large, it is easy to break, the El (total elongation) is low, and the yield ratio is high.
[0122]
No. 32 indicates A in the first continuous annealing step.₁A above the point₃Since the heating holding time (t1) at the temperature (T1) below the temperature is too long, the generation of an austenite phase having undissolved carbide as a nucleus progresses.₃It is considered that the variation in the C content in austenite is reduced even when the material is heated to a temperature above the point. Therefore, the C content in the finally obtained retained austenite is also uniform, so that the ratio of stable retained austenite increases, and the ratio of retained austenite that transforms to martensite by applying a 2% strain to the retained austenite is increased. Below the range of the present invention, the yield ratio increases.
[0123]
【The invention's effect】
According to the present invention, even with a high-strength and ultra-high-strength steel sheet of the class of 500 to 1400 MPa, a high elongation having a total elongation of 35% or more and a yield ratio of 50% or less and excellent in workability and shape freezing property. It has been possible to provide a high-strength steel sheet and a method for reliably manufacturing the steel sheet.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view showing a part of an apparatus used for saturation magnetization measurement.
FIG. 2 is a diagram illustrating a first continuous annealing step.
FIG. 3 is a diagram illustrating a second continuous annealing step or a plating step.
[Explanation of symbols]
1 4πI coil for magnetization detection
2 Sample rod
3 H coil for magnetic field detection
4 Conductor

Claims (6)

In mass%,
C: 0.06-0.6%,
Si + Al: 0.5-3%,
Mn: 0.5-3%,
P: 0.15% or less (excluding 0%),
S: 0.02% or less (including 0%)
Together with
Tempered martensite: 15% or more in area ratio to the entire structure
Ferrite: 5 to 60% in area ratio with respect to the entire structure
Retained austenite: 5% or more by volume relative to the entire structure,
Furthermore, it has a structure that may contain bainite and / or martensite, and
A high-strength steel sheet excellent in workability and shape freezing properties, wherein a ratio of retained austenite which transforms to martensite by applying a 2% strain to the retained austenite is 20 to 50%. Furthermore, in mass%,
Mo: 1% or less (excluding 0%),
Ni: 0.5% or less (excluding 0%),
Cu: 0.5% or less (excluding 0%),
Cr: 1% or less (excluding 0%)
The high-strength steel sheet according to claim 1, which contains at least one of the following. Furthermore, in mass%,
Ti: 0.1% or less (excluding 0%),
Nb: 0.1% or less (excluding 0%),
V: 0.1% or less (excluding 0%)
3. The high-strength steel sheet according to claim 2, which contains at least one of the following. Furthermore, in mass%,
Ca: 0.003% or less (excluding 0%), and / or
REM: 0.003% or less (excluding 0%)
The high-strength steel sheet according to claim 2 or 3, which contains: A method for producing a high-strength steel sheet according to any one of claims 1 to 4,
At least a hot rolling step, a first continuous annealing step, and a second continuous annealing step or plating step,
The first continuous annealing step,
A step of 10 to 300 seconds heating and holding at a temperature of A ₁ point or more _A or less _three;
Subsequently a step of 10 to 300 seconds heating and holding at a temperature of _3-point than _A; an average cooling rate and 30 ° C. / sec or more, comprising the step of cooling to a temperature below Ms point,
The second continuous annealing step or plating step,
A step of 10 to 600 seconds heating and holding at a temperature of A ₁ point or more _A or less _three;
Workability and shape freezing, comprising a step of cooling at an average cooling rate of 3 ° C./sec or more to a temperature of not less than the Ms point and not more than the Bs point; High strength steel plate manufacturing method. The method for producing a high-strength steel sheet according to claim 5, further comprising a cold rolling step after the hot rolling step and prior to the first continuous annealing step.

Images